/*
 * FreeRTOS Kernel <DEVELOPMENT BRANCH>
 * Copyright (C) 2021 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
 *
 * SPDX-License-Identifier: MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
 * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 * https://www.FreeRTOS.org
 * https://github.com/FreeRTOS
 *
 */

/*
 * Changes from V1.00:
 *
 + pxPortInitialiseStack() now initialises the stack of new tasks to the
 +    same format used by the compiler.  This allows the compiler generated
 +    interrupt mechanism to be used for context switches.
 +
 + Changes from V2.4.2:
 +
 + pvPortMalloc and vPortFree have been removed.  The projects now use
 +    the definitions from the source/portable/MemMang directory.
 +
 + Changes from V2.6.1:
 +
 + usPortCheckFreeStackSpace() has been moved to tasks.c.
 */



#include <stdlib.h>
#include "FreeRTOS.h"

/*-----------------------------------------------------------*/

/* See header file for description. */
StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
                                     TaskFunction_t pxCode,
                                     void * pvParameters )
{
    StackType_t DS_Reg = 0;
    StackType_t * pxOriginalSP;

    /* Place a few bytes of known values on the bottom of the stack.
     * This is just useful for debugging. */

    *pxTopOfStack = 0x1111;
    pxTopOfStack--;
    *pxTopOfStack = 0x2222;
    pxTopOfStack--;
    *pxTopOfStack = 0x3333;
    pxTopOfStack--;
    *pxTopOfStack = 0x4444;
    pxTopOfStack--;
    *pxTopOfStack = 0x5555;
    pxTopOfStack--;


    /*lint -e950 -e611 -e923 Lint doesn't like this much - but nothing I can do about it. */

    /* We are going to start the scheduler using a return from interrupt
     * instruction to load the program counter, so first there would be the
     * status register and interrupt return address.  We make this the start
     * of the task. */
    *pxTopOfStack = portINITIAL_SW;
    pxTopOfStack--;
    *pxTopOfStack = FP_SEG( pxCode );
    pxTopOfStack--;
    *pxTopOfStack = FP_OFF( pxCode );
    pxTopOfStack--;

    /* We are going to setup the stack for the new task to look like
     * the stack frame was setup by a compiler generated ISR.  We need to know
     * the address of the existing stack top to place in the SP register within
     * the stack frame.  pxOriginalSP holds SP before (simulated) pusha was
     * called. */
    pxOriginalSP = pxTopOfStack;

    /* The remaining registers would be pushed on the stack by our context
     * switch function.  These are loaded with values simply to make debugging
     * easier. */
    *pxTopOfStack = FP_OFF( pvParameters ); /* AX */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0xCCCC; /* CX */
    pxTopOfStack--;
    *pxTopOfStack = FP_SEG( pvParameters ); /* DX */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0xBBBB; /* BX */
    pxTopOfStack--;
    *pxTopOfStack = FP_OFF( pxOriginalSP ); /* SP */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0xBBBB; /* BP */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x0123; /* SI */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0xDDDD; /* DI */

    /* We need the true data segment. */
    __asm {
        MOV DS_Reg, DS
    };

    pxTopOfStack--;
    *pxTopOfStack = DS_Reg; /* DS */

    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0xEEEE; /* ES */

    /* The AX register is pushed again twice - don't know why. */
    pxTopOfStack--;
    *pxTopOfStack = FP_OFF( pvParameters ); /* AX */
    pxTopOfStack--;
    *pxTopOfStack = FP_OFF( pvParameters ); /* AX */


    #ifdef DEBUG_BUILD

        /* The compiler adds space to each ISR stack if building to
         * include debug information.  Presumably this is used by the
         * debugger - we don't need to initialise it to anything just
         * make sure it is there. */
        pxTopOfStack--;
    #endif

    /*lint +e950 +e611 +e923 */

    return pxTopOfStack;
}
/*-----------------------------------------------------------*/
